In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter, cream cheese, brown sugar, granulated sugar, egg, and vanilla until well combined and fluffy, about 5 minutes.

Add the flour, cornstarch, baking soda, and salt; mix just until combined, about 1 minute.

Fold in the chocolate chips or chunks until evenly distributed.

Use a 2-inch cookie scoop to form heaping mounds on a plate, flatten slightly, cover with plastic wrap, and refrigerate for at least 2 hours.

Preheat oven to 350°F. Line baking sheets with silicone mats or parchment paper.

Bake for 8 to 9 minutes until edges are set and tops are glossy. Do not overbake.

Allow cookies to cool on baking sheet for 5 minutes, then transfer to wire racks to cool completely.